A peripheral memory device which has both rapidness in access of a magnetic disk and big capacity of memory of an optical disk has been developed. The next-generation disk devices must be optical disks since they have high-speed transfer and random access, big memory capacity, storage stability of medium and durability of medium due to non-contact drive. As an optical disk medium which is subjected to access from the inner to outer radius of the disk and in the opposite direction thereto, an erasable magneto-optical disk, or a phase transition disk is desirable.
A conventional optical head for accessing the optical disk is generally structured to pile up in the order of a linear motor generating a drive force to access inner to outer radii of the optical disk, a carriage driven to move in the direction, an optical system mounted on the carriage for projecting and receiving a beam to and from the optical disk, and an actuator for actuating an objective lens to adjust focussing and tracking on the optical disk. However, its structure was a big problem on performing high-speed access.
For providing an optical head with high-speed random access, high acceleration, etc. is required to move on the inner to outer radii of the disk at high speed. To increase acceleration, it is required to reduce the weight of the movable body as much as possible and to employ an electromagnetic drive by a magnetic circuit with high efficiency.
To obtain a thinner optical head, the conventional piled-up structure in which the linear motor, the carriage, the optical system and the objective lens actuator are piled up in that order may be replaced by a parallel structure as illustrated later. However, it is difficult to obtain a good movement or transfer performance by the latter structure, although it may be thin compared to the former. In the parallel structure, since the position of the objective lens holder is offset to the center of movement of the carriage, the signal reproduction ratio CNR is lowered due to the tendency in which the position of the objective lens holder may be inclined when the carriage moves. As a result, the optical head fails to correctly read an address signal from the disk.
The optical head in the parallel structure may be provided with two drive systems, one is a linear motor which has a big thrust to quickly drive from the inner to outer radius of the disk, the other is an actuator for tracking and focusing in which the order of .+-.0.1 .mu.m is controlled for precise tracking on eccentricity of the disk. However, the trial which performs both integration of these two drive systems and thinner configuration generally has resulted in a complex structure and difficulty in assembly.
Furthermore, the optical head in the parallel structure may cause a failure in reading-out condition since the lens holder is inclined to the tangential direction by acceleration of the high-speed movement of both disk rotation and a linear motor when a lens holder for the actuator is not supported by rigid support members.